Recently, vehicle impact performance criteria for ensuring safety of passengers have become increasingly stringent. In addition, as regulations regarding carbon dioxide emissions serving as a main cause of global warming have been strengthened, improvements in fuel efficiency have been consistently demanded. For this purpose, vehicles must satisfy requirements for both sufficient frame strength and light weight.
An ultra-high strength steel sheet enables a vehicle to achieve desired strength using a thinner steel sheet.
In a conventional process for manufacturing vehicle components using an ultra-high strength steel sheet, a steel coil is initially cut to a steel sheet, which in turn is subjected to blanking into a rough shape and is then heated to an austenite region. Then, the heated steel sheet is transferred to dies, in which the steel sheet is formed to a desired shape of a component while being quenched with the formed product secured in the dies, thereby providing a high strength component with high dimensional accuracy.
FIG. 1 is a flow diagram of a conventional manufacturing method of steel products.
As shown in this figure, a steel coil 100 is unwound from a coil support 110 and passed through a cutter 120 to form a steel sheet 130. Then, the steel sheet 130 is subjected to blanking to provide a blanked steel sheet 150, which in turn is passed through a heating furnace 140 for heating the blanked steel sheet 150. The heated steel sheet 150 is then subjected to hot-forming in dies 180 to form a product 190. The dies 180 are formed with a fluid passage 185, through which a coolant or cooling water is supplied to cool the dies and the product at the same time, with the dies closed.
Here, the product must be sheared to a precise size by a shearing machine, but the ultra-high strength steel product formed by the above process has too high a strength (about 1500 MPa) to be sheared to a precise size using the shearing machine, so that too high a cutting force is required for cutting and the tool wear rate is high, thereby increasing manufacturing costs.
Further, when shearing the steel product, burrs are severely formed and increase the likelihood of cracking in a component made of the product according to notch sensitivity of a high strength material.
To solve such problems, laser cutting or water jet cutting is generally used for the shearing process. Although laser cutting or water jet cutting provides a very clean and high quality cutting surface, process time can be extended depending on the material thickness, shearing length, accuracy of dimensional tolerance, and the like. As a result, the shearing process can often cause extension in the process time of the overall manufacturing process.
Furthermore, in formation of a product having a high processing depth or a complex shape, excessive deformation occurs during hot-forming, thereby causing local damage or product defects.